When measuring workpieces having sensitive surfaces such as, for example, clay or wax models, the customary mechanical contact probes of multicoordinate measuring machines cannot be readily employed since there is the danger that the surface of the object being tested will be scratched or deformed by the probe pin. Sensing methods which operate without workpiece contact are best suited for this measurement.
Thus, it is known from West German OS No. 2,903,529 and OS No. 2,113,522 to measure objects by projecting a generally punctiform light spot onto the object at an angle to the normal to the surface and to produce an image of the light spot on a position-sensitive photoelectric detector by means of an imaging optical system which is inclined to the projection-ray path. From the position of the image of the light spot, the distance from the currently illuminated point of the workpiece is then determined by triangulation. To carry out this method, it is furthemore known to combine the projector and the imaging system into a structural group which is frequently referred to as an "optical probe". And to measure plural points on the object, this probe can be moved over the object by suitable guidance systems.
However, the object distance from the optical probe can be determined without error by this method only if there is no change in the angle which the surface of the object forms with the axes of the probe or if the probe is so guided along the workpiece that the image of the projected light spot always appears in the center of the detector. Otherwise, measurement errors occur, since the magnitude of light-spot deviation from the center position is dependent not only on the object distance but also on the angle between the plane of the object and the projection axis. And to provide for continuous guidance of the probe along the contour of the object, not only results in great expense for control technique but also limits the speed of the measurement, in view of the masses which must be accelerated.